I'm new robotic stuff, apparently I have to build a line following robot with Obstacle avoidance. I'm using a Arduino and SeedStudio motor driver to build a robot. I'm using four 1.5 Alkaline batteries to run this robot. It is working perfectly when I used it only as line following robot. But I added a ping sensor to avoid the obstacles along with line following, it is not moving at all. Interesting thing is when I connected the USB cable to Arduino along with batteries it is moving forward. So I could draw some conclusion that current required for circuit with ping sensor is enough when I connected USB and batteries. But I wann know I can increase the current in my circuit.

You reply is much appreciated. If let me know if you need more details.